Process for producing porous sintered aluminum, and porous sintered aluminum

ABSTRACT

This method for producing porous sintered aluminum includes: mixing aluminum powder with a sintering aid powder containing titanium to obtain a raw aluminum mixed powder; mixing the raw aluminum mixed powder with a water-soluble resin binder, water, and a plasticizer containing at least one selected from polyhydric alcohols, ethers, and esters to obtain a viscous composition; drying the viscous composition in a state where air bubbles are mixed therein to obtain a formed object prior to sintering; and heating the formed object prior to sintering in a non-oxidizing atmosphere, wherein when a temperature at which the raw aluminum mixed powder starts to melt is expressed as Tm (° C.), a temperature T (° C.) of the heating fulfills Tm−10 (° C.)≦T≦685 (° C.).

TECHNICAL FIELD

The present invention relates to porous sintered aluminum (porousaluminum sintered body) and the production method thereof, and theporous sintered aluminum is particularly suitable as a current collectorfor a lithium-ion secondary battery and an electrical double layercapacitor.

The present application claims priority on Japanese Patent ApplicationNo. 2009-082498 filed on Mar. 30, 2009, the content of which isincorporated herein by reference.

BACKGROUND ART

Recently, aluminum foil has generally been used as a current collectorfor positive electrodes of a lithium-ion battery and an electricaldouble layer capacitor. In addition, such a battery and a capacitor havebeen used in electrical vehicles and the like in recent years, and theelectrode with the current collector in the battery and the capacitorhas been required to have a higher output and a higher energy densitywith the broadening of the usage purposes. As described in PatentDocuments 1 and 2, porous aluminum body which includes open pores havinga three-dimensional network structure has been known as a currentcollector for an electrode.

As a method for producing this porous aluminum body, a foam-meltingmethod has been known as disclosed in Patent Document 3. In thisfoam-melting method, a thickener is added to a melted aluminum so as toincrease the viscosity, and then titanium hydride as a foaming agent isadded thereto. While foaming the melted aluminum by utilizing hydrogengas generated in a thermal decomposition reaction of the titaniumhydride, the melted aluminum is solidified. However, foamed aluminumobtained by this method includes large closed pores having sizes ofseveral millimeters.

There are other methods, and the following method is exemplified as asecond method. Aluminum is pressed into a casting mold having a core ofsponge urethane, and a hollow cavity formed after burning off theurethane is filled with the aluminum. Thereby, foamed aluminum having asponge skeleton is obtained. According to this method, foamed aluminumis obtained which includes open pores having pore diameters that fulfill40 PPI or smaller, that is, 40 cells or less per inch (pore diameters ofabout 600 μm or larger).

The following method is exemplified as a third method. As disclosed inPatent Document 4, aluminum alloy is infiltrated into a reinforcingmaterial made of hollow ceramics by the pressure infiltration method;and thereby, foamed aluminum is obtained which includes closed poreshaving pore diameters of 500 μm or smaller in accordance with thedimension of the reinforcing material.

The following method is exemplified as a fourth method. As disclosed inPatent Document 5, a mixed powder of AlSi alloy powder and TiH₂ powderis sandwiched between aluminum plate materials, and the mixed powder isheated and rolled in such a state. Thereby, aluminum is foamed due tothe decomposition of the TiH₂ powder. The foamed aluminum obtained bythis method includes pores having large pore diameters of severalmillimeters.

The following method is exemplified as a fifth method. As disclosed inPatent Document 6, aluminum is mixed with metal of which eutectictemperature with aluminum is lower than the melting point of aluminum,and the mixture is heated at a temperature which is higher than theeutectic temperature and lower than the melting point of aluminum.Foamed aluminum obtained by this method has a porosity of about 40%which is low, although the pore diameters can be reduced by this method.Therefore, in the case where the foamed aluminum is used as a currentcollector, an amount of cathode active material or anode active materialinfiltrated into the pores of the foamed aluminum is small, and thedesired high output and high energy density cannot be achieved.

Accordingly, among the aforementioned foam-melting method and the secondto fifth methods, the second method in which aluminum is pressed into acasting mold having a core of sponge urethane, is employed as a methodof producing foamed aluminum including fine open pores which can attainthe high output and high energy density.

However, even in the second method, it is necessary to use spongeurethane having a fine microporous structure in order to further reducethe pore diameters of the open pores. In the case where such a spongeurethane is used, the flow of aluminum worsens; and thereby, aluminumcannot be press-filled into the hollow, or the casting pressure becomesexcessively high. Therefore, it is difficult to manufacture foamedaluminum which includes pores having pore diameters that fulfill smallerthan 40 PPI.

A slurry foaming method is disclosed in Patent Document 7 as a methodfor producing foamed metal which has a high porosity and includes openpores having small diameters and uniform dimensions, in which aplurality of fine open pores are uniformly arranged. In the slurryfoaming method, foamable slurry containing metal powder and a foamingagent is foamed, and dried. Thereafter, the foamed and dried slurry issintered. According to this method, if a raw material powder which canbe sintered is prepared, it is possible to easily manufacture foamedmetal which has a high porosity and includes open pores having uniformdimensions and arbitrary pore diameters that fulfill about 10 PPI toabout 500 PPI, that is, within a pore diameter range of 2.5 mm to 50 μm.Here, the slurry foaming method means a method for producing foamedmetal in which foaming is conducted by containing the foaming agent asdescribed above or foaming is conducted by injecting gas and stirring,and the foamable slurry as described above is sintered in the foamedstate.

However, conventionally, it is difficult to manufacture foamed aluminumby the slurry foaming method.

The reason is as follows. In the slurry foaming method, metal powder issintered by free sintering which is performed without applying stresssuch as a compression stress or the like; and thereby, foamed metal isobtained. However, the surface of aluminum powder is covered with densealuminum oxide film having a thickness of several nanometers to severaltens of nanometers, and this aluminum oxide film inhibits the sinteringregardless of being solid phase sintering or a liquid phase sintering.Therefore, it is difficult to proceed sintering by the free sintering;and as a result, uniform foamed aluminum cannot be obtained by theslurry foaming method.

Therefore, a method can be exemplified which employs a combination ofthe slurry foaming method and the aforementioned fifth method, as amethod for sintering the aluminum powder by the free sintering.According to this method, copper powder is prepared as a metal whoseeutectic temperature with aluminum is lower than the melting point ofaluminum, and the copper powder and a foaming agent are mixed withaluminum. Then the mixture is heated and sintered at a temperature whichis higher than the eutectic temperature and lower than the melting pointof aluminum. Thereby, foamed aluminum is obtained. However, liquiddroplets of aluminum ooze out of the surface, and the liquid dropletsare solidified; and as a result, a plurality of aluminum lumps havingsemispherical shapes are formed. In particular, in the case where thefoamed aluminum has a thin plate shape, the formation of the aluminumlumps occurs remarkably as shown in FIG. 4, and it is not possible tomanufacture desired uniform foamed aluminum.

PRIOR ART DOCUMENT Patent Documents

-   [Patent Document 1] Japanese Patent No. 3591055-   [Patent Document 2] Japanese Unexamined Patent Application,    Publication No. 2009-43536-   [Patent Document 3] Japanese Unexamined Patent Application,    Publication No, H08-209265-   [Patent Document 4] Japanese Unexamined Patent Application,    Publication No. 2007-238971-   [Patent Document 5] Published Japanese Translation No. 2003-520292    of the PCT International Publication-   [Patent Document 6] Japanese Examined Patent Application,    Publication No. S61-48566-   [Patent Document 7] Japanese Patent No. 3535282

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention is made in view of the above circumstances, andthe present invention aims to provide a method for producing poroussintered aluminum, and in accordance with this method, it is possible toobtain uniform foamed aluminum which has a high porosity and includesopen pores having pore diameters that fulfill 40 PPI or greater, thatis, pore diameters of 600 μm or smaller, and uniform dimensions. Inaddition, the present invention also aims to provide porous sinteredaluminum, and the porous sintered aluminum can be suitably used as acurrent collector for a positive electrode of a battery or a capacitorwhich requires high output and high energy density.

Means for Solving the Problems

The present inventors found that there were conditions under which itwas possible to perform sintering even by free sintering withoutgenerating lumps of liquid droplets if an aluminum powder was mixed witha sintering aid powder containing titanium and the mixture was heatedand sintered at a temperature within a predetermined range, and thepresent inventors completed the present invention.

That is, the method for producing porous sintered aluminum of thepresent invention includes: mixing aluminum powder with a sintering aidpowder containing titanium to obtain a raw aluminum mixed powder; mixingthe raw aluminum mixed powder with a water-soluble resin binder, water,and a plasticizer containing at least one selected from polyhydricalcohols, ethers, and esters to obtain a viscous composition; drying theviscous composition in a state where air bubbles are mixed therein toobtain a formed object prior to sintering; and heating the formed objectprior to sintering in a non-oxidizing atmosphere. When a temperature atwhich the raw aluminum mixed powder starts to melt is expressed as Tm (°C.), then a temperature T (° C.) of the heating fulfills Tm−10 (°C.)≦T≦685 (° C.).

Here, the non-oxidizing atmosphere means an atmosphere in which rawaluminum mixed powder is not oxidized, and examples thereof include aninert atmosphere, and a reducing atmosphere. In addition, theaforementioned heating temperature is not the temperature of the rawaluminum mixed powder, that is, the heating temperature means not themeasured reaction temperature of the raw aluminum mixed powder but theholding temperature of the surrounding around the raw aluminum mixedpowder.

The method of the present invention is a method of subjecting newaluminum powder to free sintering, and the thus obtained sintered object(sintered body) is a porous sintered object (sintered body) in which thecompounds of aluminum and titanium are dispersed and distributeduniformly and two or more pores are opened per linear length of 100 μm.

Moreover, with regard to the sintering method in the present invention,sintering is combined with a known slurry foaming method. As a result,the present invention relating to a production method was completedwhich enables to produce uniform foamed porous sintered aluminum thathas high porosity and includes fine open pores having pore diameters ofsmaller than 600 μm and uniform dimensions.

That is, the present invention is characterized by the following steps.The raw aluminum mixed powder is mixed with a water-soluble resinbinder, water, and a plasticizer containing at least one selected frompolyhydric alcohols, ethers, and esters to obtain a viscous composition.The viscous composition is dried in a state where air bubbles are mixedtherein to obtain a formed object prior to sintering. Then, the formedobject prior to sintering is heated in a non-oxidizing atmosphere.

In those steps, the formed object prior to sintering which is foamed isproduced from the raw aluminum mixed powder by a known slurry foamingmethod, and the formed object prior to sintering is sintered to producethe porous sintered aluminum. Thereby, a porous body is obtained whichincludes two different kinds of pores. The two different kinds of poresincludes pores surrounded by sponge skeletons formed by the slurryfoaming method and pores formed in the sponge skeleton itself.

In the method for producing porous sintered aluminum of the presentinvention, an average particle diameter of the aluminum powder may be ina range of 2 to 200 μm.

When an average particle diameter of the sintering aid powder isexpressed as r (μm), and a mixing ratio of the sintering aid powder isexpressed as W (% by mass), r and W may fulfill 1 (μm)≦r≦30 (μm), 1 (%by mass)≦W≦20 (% by mass), and 0.1≦W/r≦2.

The sintering aid powder may be either one or both of titanium andtitanium hydride.

Non-water-soluble hydrocarbon system organic solvent containing 5 to 8carbons may be added to the viscous composition.

The water-soluble resin binder may be contained at a content in a rangeof 0.5% to 7% of the quantity (mass) of the raw aluminum mixed powder.

Surfactant may be added to the raw aluminum mixed powder at a content ina range of 0.02 to 3% of the quantity (mass) of the raw aluminum mixedpowder.

The viscous composition may be extended to have a thickness of 0.05 mmto 5 mm, and then the viscous composition may be dried to produce theformed object prior to sintering as a plate-like formed object.

The porous sintered aluminum of the present invention includes metalskeletons having a three-dimensional network structure of perforatedsintered metal. Pores exist between the metal skeletons. Al—Ti compoundis dispersed in the perforated sintered metal, and the pores are formedat an amount of 20 or more pores per linear length of 1 cm, and thereby,an overall porosity is in a range of 70 to 99%.

EFFECTS OF THE INVENTION

In accordance with the method for producing porous sintered aluminum ofthe present invention, the aluminum powder is mixed with the sinteringaid powder containing titanium to obtain the raw aluminum mixed powder,and then the raw aluminum mixed powder is heated at the temperature Twhich fulfills Tm−10 (° C.)≦T≦685 (° C.). Thereby, it is possible toobtain porous sintered aluminum which includes two or more open poresper linear length of 100 μm.

Here, the reason that the heating temperature is limited to not lessthan Tm−10 (° C.) is because the temperature at which the aluminumpowder contained in the raw aluminum mixed powder starts to react withthe sintering aid powder containing titanium is Tm−10 (° C.). Themelting point of aluminum is described as Tm because industrially usedaluminum contains impurities such as iron and silicon and the meltingpoint thereof becomes lower than 660° C., which is the melting point ofpure aluminum. On the other hand, the reason that the heatingtemperature is limited to 685° C. or lower is because aluminum lumpshaving liquid droplet shapes are generated in the sintered body in thecase where the mixture is heated and maintained at a temperature whichis higher than 685° C.

At this time, the raw aluminum mixed powder is mixed with thewater-soluble resin binder, water, and the plasticizer to obtain theviscous composition, and the viscous composition is dried in a statewhere air bubbles are mixed therein to obtain the formed object prior tosintering. Then, and the formed object prior to sintering is heated at atemperature in the above-mentioned range. As a result, since the formedobject prior to sintering has a sponge skeleton structure(three-dimensional skeleton structure, foamed skeleton structureincluding open pores), the obtained sintered body is porous aluminumwhich includes two different kinds of pores including pores surroundedby the sponge skeletons and pores formed in the sponge skeleton itself.

The aluminum powder is provided such that the viscous composition has aviscosity at a level at which the composition can be formed into adesired shape and the formed object prior to sintering obtained bydrying the viscous composition in a state where air bubbles are mixedtherein has a desired handling strength. That is, in the case where theaverage particle diameter is small, it is necessary to increase thequantity (mass) of water-soluble resin coupler with respect to thequantity (mass) of the aluminum powder so as to secure the viscosity andthe handling strength. However, in the case where the quantity (mass) ofthe water-soluble resin coupler becomes large, the amount of carbonremaining in the aluminum is increased during the formed object prior tosintering is heated, and the remained carbon inhibits the sinteringreaction. On the other hand, in the case where the particle diameter ofthe aluminum powder is excessively large, the strength of the poroussintered body is lowered.

Therefore, it is preferable that the average particle diameter of thealuminum powder be in a range of 2 μm or greater; and thereby, theinhibition of the sintering reaction due to the large quantity (mass) ofwater-soluble resin coupler is prevented. Moreover, it is preferablethat the average particle diameter of the aluminum powder be 200 μm orsmaller; and thereby, the strength of the porous sintered body issecured. More preferably, the average particle diameter of the aluminumpowder is set to be in a range of 7 μm to 40 μm.

With regard to the sintering aid powder, it is preferable that theaverage particle diameter r (μm) and the mixing ratio W (% by mass)fulfill 1 (μm)≦r≦30 (μm), 0.1 (% by mass)≦W≦20 (% by mass), and0.1≦W/r≦2.

The reason is as follows. In the case where the mixing ratio W of thesintering aid powder exceeds 20% by mass, sintering aid particles havecontacts with each other in the raw aluminum mixed powder; and thereby,the reaction heat between aluminum and titanium cannot be controlled,and a desired porous sintered body cannot be obtained. Therefore, it ispreferable to fulfill 0.1 (% by mass)≦W≦20 (% by mass). In addition, itis more preferable to fulfill 1 (% by mass)≦W≦20 (% by mass).

Even in the case where the mixing ratio fulfills 0.1 (% by mass)≦W≦20 (%by mass), the reaction heat between aluminum and titanium becameexcessively high in some cases depending on the particle diameter of thesintering aid powder. In these cases, the temperature of melted aluminumdue to the reaction heat further rose; and thereby, the viscositythereof was lowered. As a result, liquid droplets were generated in somecases.

In view of these, test pieces were manufactured under variousconditions, and the test pieces were observed by an electron microscope.As a result of the observation, it was found that only a surface layerportion having a substantially constant thickness from the exposedsurface side of the titanium particle reacted with aluminum in the casewhere the amount of heat generation was controlled to be in a rangecontrollable by the mixing ratio of titanium and the particle diameterof titanium. From the experimental results, it was found that theconditions of 1 (μm)≦r≦30 (μm) and 0.1 (% by mass)≦W/r≦2 (% by mass) arepreferable in order to prevent the occurrence of liquid droplets.

Hereinafter, the meaning of 0.1≦W/r≦2 in the case of using titanium asthe sintering aid powder will be described. When the average particlediameter of titanium is expressed as r, the number of titanium-particlesis expressed as N, the additive quantity (mass) of titanium is expressedas w, the specific weight of titanium is expressed as D, and thereduction amount in the titanium particle diameter due to the reactionwith aluminum is expressed as d, the reaction heat amount Q fulfillsQ∝4π²dN since the reaction heat amount Q is proportional to the volumeof reacted titanium. Moreover, since the additive amount of the titaniumparticles is calculated as a product of the average volume of onetitanium particle and the number of titanium particles, w=4/3πr³DN isobtained. Accordingly, if the latter equation is substituted into theformer equation, Q∝3wd/rD is obtained. Here, Q∝w/r is further obtainedbased on the fact that 3/D is a constant and the observation result thatd is substantially constant regardless of the sintering conditions.Therefore, the range of W/r in which the liquid droplets are notgenerated is experimentally determined and the range is limited asdescribed above. Thereby, the generation of liquid droplets due to theexcessively high reaction heat between aluminum and titanium isprevented.

In addition, the titanium hydride as the sintering aid powder containstitanium at a content of 95% by mass or greater, and dehydrogenation ofthe titanium hydride occur at a temperature of 470 to 530° C. to convertinto titanium. Therefore, the titanium hydride is thermally decomposedinto titanium by the aforementioned heating. Accordingly, it is possibleto enhance the reaction efficiency with the aluminum powder by usingtitanium and/or titanium hydride as the sintering aid powder.

Furthermore, the viscous composition can be foamed by the addition ofnon-water-soluble hydrocarbon system organic solvent containing 5 to 8carbons; and thereby, air bubbles can be mixed into the viscouscomposition.

In the case where the contained amount of the water-soluble couplerexceeds 7% of the quantity (mass) of the raw aluminum mixed powder, theamount of carbon remaining in the formed object prior to sintering isincreased during heating, and the remained carbon inhibits the sinteringreaction. On the other hand, in the case where the contained amount ofthe water-soluble coupler is less than 0.5%, it is not possible tosecure the handling strength of the formed object prior to sintering.Therefore, it is preferable that the water-soluble coupler be containedat a content in a range of 0.5% to 7% of the quantity (mass) of the rawaluminum mixed powder.

In addition, it is possible to effectively generate air bubbles byadding a surfactant to the raw aluminum mixed powder. In the case wherethe added amount of this surfactant is set to be in a range of 0.02% orgreater of the quantity (mass) of the raw aluminum mixed powder, it ispossible to achieve an effect due to the addition of the surfactant. Inthe case where the added amount of the surfactant is set to be in arange of 3% or smaller of the quantity (mass) of the raw aluminum mixedpowder, it is also possible to prevent the inhibition of the sinteringreaction due to the increased amount of carbon remaining in the formedobject prior to sintering.

In the case where the viscous composition is extended to have athickness of 0.05 mm to 5 mm to form the formed object prior tosintering as a plate-shaped (plate-like) formed object, it is possibleto obtain porous sintered aluminum by sintering the plate-shaped formedobject, and the porous sintered aluminum is suitable as a currentcollector for a lithium-ion secondary battery or an electrical doublelayer capacitor.

Even in the case where the porous sintered aluminum of the presentinvention is used as a current collector of a positive electrode and theporous sintered aluminum is coiled around the outer circumference of acylindrical battery element, cathode active material is not peeled off.In addition, even when expansion and contraction of the active materialoccur due to charging and discharging, the cathode active material isnot easily peeled off. Therefore, the porous sintered aluminum of thepresent invention can be stably used as the current collector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM photograph of foamed aluminum in Example 1.

FIG. 2 is a partially enlarged SEM photograph of FIG. 1.

FIG. 3 is an SEM photograph of foamed aluminum in Comparative Example 1.

FIG. 4 is a photograph of foamed aluminum obtained by a combined methodwhich includes a fifth method in the conventional art as a method ofperforming free sintering of aluminum powder and a slurry foamingmethod.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, one embodiment of the method for producing porous sinteredaluminum according to the present invention will be described.

The method for producing aluminum of the present embodiment includes thefollowing steps:

a process of providing raw aluminum mixed powder in which aluminum poweris mixed with titanium and/or titanium hydride to obtain raw aluminummixed powder;

a process of providing a viscous composition in which the raw aluminummixed powder is mixed with water-soluble resin coupler, water, andplasticizer to provide a slurry viscous composition;

a process prior to sintering in which the viscous composition is driedin a state where air bubbles are mixed therein to obtain a formed objectprior to sintering; and

a sintering process in which the formed object prior to sintering isheated at a temperature T that fulfills Tm−10 (° C.)≦heating temperatureT≦685 (° C.) in a non-oxidizing atmosphere.

Here, Tm (° C.) represents the temperature at which the raw aluminummixed powder starts to melt.

In the process of providing the raw aluminum mixed powder, an aluminumpowder having an average particle diameter of 2 to 200 μm is used. Thereason is as follows. In the case where the average particle diameter issmall, it is necessary to add a large amount of water-soluble resincoupler to the aluminum powder in order that the viscous composition hasa viscosity at which the viscous composition can be formed in to adesired shape and the formed object prior to sintering has a handlingstrength. However, in the case where a large amount of water-solubleresin coupler is added, an amount of carbon remaining in the aluminum isincreased when the formed object prior to sintering is heated, and theremained carbon inhibits the sintering reaction. On the other hand, inthe case where the particle diameter of the aluminum powder isexcessively large, the strength of the foamed aluminum is lowered.Accordingly, as described above, the aluminum powder having the averageparticle diameter in a range of 2 to 200 μm is used, and the averageparticle diameter is more preferably in a range of 7 to 40 μm.

The aluminum powder is mixed with titanium and/or titanium hydride. Thereason is as follows. In the case where the aluminum powder is mixedwith titanium and the formed object prior to sintering is heated at theheating temperature T which fulfills Tm−10 (° C.)≦heating temperatureT≦685 (° C.), it is possible to perform free sintering of aluminumwithout generating lumps of liquid droplets. In addition, titaniumhydride (TiH₂) contains titanium at a content of 47.88 (molecular weightof titanium)/(47.88+1 (molecular weight of hydrogen)×2), which is 95% bymass or greater, and dehydrogenation of the titanium hydride occurs at atemperature of 470 to 530° C. to convert into titanium. Therefore, thetitanium hydride is thermally decomposed into titanium by theaforementioned heating. Accordingly, it is possible to perform freesintering of aluminum without generating lumps of liquid droplets evenin the case where the titanium hydride is mixed thereinto.

Here, when the average particle diameter of titanium or titanium hydrideis expressed as r (μm), and the mixing ratio of titanium or titaniumhydride is expressed as W (% by mass), the following equations arefulfilled: 1 (μm)≦r≦30 (μm), 0.1 (% by mass)≦W≦20 (% by mass), and0.1≦W/r≦2. For example, in the case where the titanium hydride powderhas an average particle diameter of 4 μm, 0.1≦W/4≦2 is to be fulfilled;and therefore, the mixing ratio W becomes in a range of 0.4 to 8% bymass. In the case where the titanium powder has an average particlediameter of 20 μm, 0.1≦W/20≦2 is to be fulfilled; and therefore; themixing ratio W becomes in a range of 2 to 40% by mass. However, since0.1 (% by mass)≦W≦20 (% by mass) is to be fulfilled, the mixing ratiobecomes in a range of 2 to 20% by mass.

The average diameter of titanium hydride is set to fulfill 0.1 (μm)≦r≦30(μm), the average diameter is preferably set to fulfill 1 (μm)≦r≦30 (μm)and is more preferably set to fulfill 4 (μm)≦r≦20 (μm). The reason is asfollows. In the case where the average diameter is 1 μm or smaller,there is a concern of spontaneous combustion. After sintering, thetitanium hydride becomes titanium grains covered with a compound ofaluminum and titanium. In the case where the average particle diameterexceeds 30 μm, the compound phase of aluminum and titanium is easilypeeled off from the titanium grains; and thereby, a desired strength ofthe sintered body cannot be obtained.

The reason that 0.1 (% by mass)≦W≦20 (% by mass) is set is as follows.In the case where the mixing ratio W of the sintering aid powder exceeds20% by mass, the sintering aid particles contact with each other in theraw aluminum mixed powder; and thereby, the reaction heat betweenaluminum and titanium cannot be controlled, and a desired poroussintered body cannot be obtained.

Even in the case where the mixing ratio fulfills 0.1 (% by mass)≦W≦20 (%by mass), the reaction heat between aluminum and titanium becameexcessively high in some cases depending on the particle diameter of thesintering aid powder. In these cases, the temperature of melted aluminumdue to the reaction heat further rose; and thereby, the viscositythereof was lowered. As a result, liquid droplets were generated in somecases.

In view of these, test pieces were manufactured under variousconditions, and the test pieces were observed by an electron microscope.As a result of the observation, it was found that only a surface layerportion having a substantially constant thickness from the exposedsurface side of the titanium particle reacted with aluminum in the casewhere the amount of heat generation was controlled to be in a rangecontrollable by the mixing ratio of titanium and the particle diameterof titanium. From the experimental results, it was found that theconditions of 1 (μm)≦r≦30 (μm) and 0.1 (% by mass)≦W/r≦2 (% by mass) arepreferable in order to prevent the occurrence of liquid droplets.

Hereinafter, the meaning of 0.1≦W/r≦2 in the case of using titanium asthe sintering aid powder will be described. When the average particlediameter of titanium is expressed as r, the number of titanium particlesis expressed as N, the additive quantity (mass) of titanium is expressedas w, the specific weight of titanium is expressed as D, and thereduction amount in the titanium particle diameter due to the reactionwith aluminum is expressed as d, the reaction heat amount Q fulfillsQ∝4πr²dN since the reaction heat amount Q is proportional to the volumeof reacted titanium. Moreover, since the additive amount of the titaniumparticles is calculated as a product of the average volume of onetitanium particle and the number of titanium particles, w=4/3πr³DN isobtained. Accordingly, if the latter equation is substituted into theformer equation, Q∝3wd/rD is obtained. Here, Q∝w/r is further obtainedbased on the fact that 3/D is a constant and the observation result thatd is substantially constant regardless of the sintering conditions.

Therefore, the range of W/r in which the liquid droplets are notgenerated is experimentally determined and the range is limited asdescribed above. Thereby, the generation of liquid droplets due to theexcessively high reaction heat between aluminum and titanium isprevented.

The following components are added to the raw aluminum mixed powder inthe process of providing the viscous composition: at least one kindselected from polyvinyl alcohol, methylcellulose, and ethylcellulose asa water-soluble resin coupler; at least one kind selected frompolyethyleneglycol, glycerin, and di-N-buthyl phthalate as aplasticizer; distillated water; and alkylbetaine as a surfactant.

In the case where at least one kind selected from polyvinyl alcohol,methylcellulose, and ethylcellulose is used as the water-soluble resincoupler, a relatively small additive amount is sufficient. Therefore,the additive amount (ratio) thereof is set to be in a range of 0.5% to7% of the quantity (mass) of the raw aluminum mixed powder. In the casewhere the additive amount of the water-soluble resin coupler exceeds 7%of the quantity (mass) of the raw aluminum mixed powder, the amount ofcarbon remaining in the formed object prior to sintering is increasedduring heating, and the remained carbon inhibits the sintering reaction.In the case where the additive amount of the water-soluble resin coupleris less than 0.5%, the handling strength of the formed object prior tosintering cannot be secured.

Alkylbetain is added at an amount (ratio) of 0.02% to 3% of the quantity(mass) of the raw aluminum mixed powder. In the case where the amount(containing ratio) is set to be 0.02% or higher of the quantity (mass)of the raw aluminum mixed powder, air bubbles are effectively generatedduring mixing a non-water-soluble hydrocarbon system organic solventwhich will be described later. By setting the amount (containing ratio)to be 3% or lower, the inhibition of the sintering reaction due to theincreased amount of carbon remaining in the formed object prior tosintering can be prevented.

After kneading the mixture, foaming is performed by further mixing thenon-water-soluble hydrocarbon system organic solvent containing 5 to 8carbons; and thereby, a viscous composition including air bubbles mixedthereinto is prepared. As the non-water-soluble hydrocarbon systemorganic solvent containing 5 to 8 carbons, at least one kind selectedfrom pentane, hexane, heptane, and octane can be used.

Next, in the step prior to sintering, a strip-shaped polyethylene sheetis prepared of which the surface is coated with separating compound, andthe viscous composition is extended to have a thickness of 0.05 mm to 5mm by coating the viscous composition on the surface of the strip-shapedpolyethylene sheet. Then, the temperature and moisture of thesurrounding (circumferential atmosphere) are controlled for a specifictime period so as to make the dimensions of air bubbles uniform.Thereafter, the resulting object is dried at a temperature of 70° C. byan air dryer. Here, the viscous composition is coated by a doctor blademethod, a slurry extrusion method, or a screen printing method.

The dried viscous composition is peeled off from the polyethylene sheet,and then, if necessary, the dried viscous composition is cut out into apredetermined shape such as a circle having a diameter of 100 mm.Thereby, the formed object prior to sintering is obtained.

Next, in the sintering process, zirconia spinkle powder is spread on analumina setter, and the formed object prior to sintering is placed onthe alumina setter. Then, pre-sintering is performed by holding theformed object prior to sintering at 520° C. for one hour in an argonatmosphere whose dew point is −20° C. or lower. Thereby, thewater-soluble resin coupler, a binder solution of the plasticizercomponent, the distillated water, and alkylbetaine are evaporated(removal of binder). In addition, dehydrogenation proceeds in the casewhere titanium hydride is used as the sintering aid powder.

Thereafter, the formed object prior to sintering which is pre-sinteredis heated at a heating temperature T which fulfills Tm−10 (° C.)≦heatingtemperature T≦685 (° C.) to obtain foamed aluminum.

This is based on the following reason. It is considered that thereaction between aluminum and titanium starts by heating the formedobject prior to sintering up to the melting temperature Tm (° C.).However, aluminum contains a very small amount of eutectic alloyelements such as Fe, Si, and the like as impurities in practice; andthereby, the melting point thereof is lowered. Therefore, it isconsidered that the reaction between aluminum and titanium starts byheating up to Tm−10 (° C.) and foamed aluminum is formed. In practice,the melting point of aluminum is 660° C.; however, the melting starttemperature of an atomized powder having a purity of about 98% to 99.7%,which is marketed as a pure aluminum powder, is about 650° C.

On the other hand, in the case where the temperature reaches 665° C.which is the peritectic temperature of aluminum and titanium, and themelting latent heat is further input, the sintered aluminum (aluminumsintered body) is melted. Therefore, it is necessary to keep thetemperature of a furnace atmosphere in a range of 685° C. or lower.

In addition, it is necessary to perform the heating of the sinteringprocess in a non-oxidizing atmosphere in order to suppress the growth ofoxide layers on the aluminum particle surface and the titanium particlesurface. However, the oxide layers on the aluminum particle surface andthe titanium particle surface do not remarkably grow even in the case ofheating in the air under the conditions where the heating temperature is400° C. or lower and the holding time is about 30 minutes. Therefore,the formed object prior to sintering may be heated and held at atemperature in a range of 300° C. to 400° C. for about 10 minutes in theair (removal of binder), and then the formed object prior to sinteringmay be heated at a predetermined temperature in an argon atmosphere.

The thus obtained foamed aluminum includes metal skeletons having athree-dimensional network structure of perforated sintered metal(perforated metal sintered body), and pores are included between themetal skeletons. In addition, Al—Ti compound is dispersed in theperforated sintered metal, 20 or more pores are formed per linear lengthof 1 cm, and the foamed aluminum has an overall porosity of 70 to 90%and is suitably used as a current collector of a lithium-ion secondarybattery or an electrical double layer capacitor.

The present invention is not limited to the aforementioned embodiments,and a sintering aid powder other than titanium and titanium hydride maybe used as long as a sintering aid powder which contains titanium as asintering aid element is used.

In the method for producing foamed aluminum of the present embodiment,the aluminum powder is mixed with the titanium and/or titanium hydrideas the sintering aid powder to prepare the raw aluminum mixed powder inthe process of providing the raw aluminum mixed powder. Then, theviscous composition produced in the process of providing the viscouscomposition is foamed, and the viscous composition is heated at atemperature T which fulfills Tm−10 (° C.)≦T≦685 (° C.) in the sinteringprocess. Thereby, it is possible to obtain uniform foamed aluminum whichhas a high porosity and includes two different kinds of pores. The twodifferent kinds of pores includes pores which are surrounded by thesponge skeletons and have pore diameters of less than 600 μm and uniformdimensions, and fine pores, which are formed in the sponge skeletonitself at an amount of two or more pores per linear length of 100 μm.The titanium hydride contains titanium at a content of 95% by mass orgreater and dehydrogenation of the titanium hydride occurs at atemperature of 470 to 530° C. to convert into titanium. Therefore, thetitanium hydride is thermally decomposed into titanium by theaforementioned heating. Accordingly, it is considered that the titaniumhydride contributes to producing foamed aluminum by the aforementionedheating in the same manner as titanium.

Moreover, the pre-sintering process is provided between the process ofproviding the viscous composition and the sintering process, and in thepre-sintering process, the strip-shaped polyethylene sheet is preparedof which the surface is coated with separating compound, and the viscouscomposition is extended to have a thickness of 0.05 mm to 5 mm bycoating the viscous composition on the strip-shaped polyethylene sheetto obtain the formed object prior to sintering having a predeterminedshape. In this case, by performing the sintering process as the postprocessing, it is possible to obtain foamed aluminum which is suitablyused as a current collector of a lithium-ion secondary battery or anelectrical double layer capacitor.

EXAMPLES Examples 1 to 16

Al powders having average particle diameters of 2.1 μm, 9.4 μm, 24 μm,87 μm, and 175 μm, Ti powders having average particle diameters of 9.8μm, 24 μm, and 42 μm, and TiH₂ powders having average particle diametersof 4.2 μm, 9.1 μm, and 21 μm were prepared. Then, in accordance with theaforementioned embodiment, the Al powder was mixed with the Ti powderand/or the TiH₂ powder at the ratios shown in Table 1 to prepare rawaluminum mixed powders 1 to 10, and binder solutions 1 to 5 having thecompounding compositions shown in Table 2 were prepared. They werekneaded with a non-water-soluble hydrocarbon system organic solvent atthe ratios shown in Table 3 to manufacture viscous compositions ofExamples 1 to 16.

TABLE 1 Raw aluminum mixed powder Aluminum powder Sintering aid powderComposition (% by weight) Average Mixing ratio Average Al and particle(% by particle Sintering inevitable diameter weight) diameter Aluminumaid Fe Si Ni Mg Cu impurities (μm) Ti TiH₂ (μm) r powder powder W W/rRaw aluminum 0.15 0.05 0.01 — — remainder 24 0 100   9.1 remainder 10.11 mixed powder 1 of the present invention Raw aluminum 0.15 0.05 0.01— — remainder 24 0 100 21 remainder 5 0.24 mixed powder 2 of the presentinvention Raw aluminum 0.15 0.05 0.01 — — remainder 24 0 100 21remainder 15  0.71 mixed powder 3 of the present invention Raw aluminum0.15 0.05 0.01 — — remainder 24 0 100   9.1 remainder 10  1.1 mixedpowder 4 of the present invention Raw aluminum 0.15 0.05 0.01 — —remainder 24 0 100   4.2 remainder 5 1.2 mixed powder 5 of the presentinvention Raw aluminum 0.15 0.05 0.01 — — remainder 24 0 100   2.8remainder 5 1.8 mixed powder 6 of the present invention Raw aluminum0.16 0.08 — — — remainder   9.4 50 50 23 remainder   0.5 0.022 mixedpowder 7 of the present invention Raw aluminum 0.18 0.06 0.01 0.4 1.6remainder 87 100 0 24 remainder 1 0.042 mixed powder 8 of the presentinvention Raw aluminum 0.2 0.3 1.6 0.4 0.1 remainder 175  100 0 23remainder 5 0.22 mixed powder 9 of the present invention Raw aluminum0.2 0.05 — — — remainder   2.1 0 100   4.2 remainder 1 0.24 mixed powder10 of the present invention Comparative raw 0.11 0.05 — — — remainder 220*¹ 100 0 24 remainder 5 0.21 aluminum mixed powder 31 Comparativeraw 0.15 0.05 0.01 — — remainder 24 0 100 21 remainder   0.1 0.005*²aluminum mixed powder 32 Comparative raw 0.15 0.05 0.01 — — remainder 240 100   4.2 remainder 15  3.6*² aluminum mixed powder 33 Comparative raw0.15 0.05 0.01 — — remainder 24 0 100 21 remainder  25*² 1.2 aluminummixed powder 34 Comparative raw 0.15 0.05 0.01 — — remainder 24 100 0  42*² remainder 15  0.36 aluminum mixed powder 35 *¹out of the scope ofClaim 3; average particle diameter of aluminum powder: 2 μm to 200 μm*²out of the scope of Claim 4; average particle diameter and mixingratio of sintering aid powder: 1 ≦ r ≦ 30 and 0.01 ≦ W/r ≦ 2

TABLE 2 Compounding composition of binder solution (% by weight)Water-soluble resin coupler Plasticizer Surfactant MC EC PVA Gr PEG ABWater Binder 5 — — 3 3 0.1 remainder solution 1 Binder 0.1 2.9 — 3 3 0.5remainder solution 2 Binder 0.2 — 4.8 1 5 2 remainder solution 3 Binder9 — — 7 5 0.5 remainder solution 4 Binder 5 — — 3 3 5 remainder solution5 MC: methylcellulose EC: ethylcellulose PVA: polyvinyl alcohol Gr:glycerin PEG: polyethyleneglycol AB: alkylbetaine

TABLE 3 Components of viscous composition Non-water-soluble Ratio of Rawaluminum hydrocarbon system waret- mixed powder A Binder solutionorganic solvent soluble Ratio of Mixing ratio Mixing ratio Mixing ratioresin coupler surfactant to Type (% by weight) Type (% by weight) Type(% by weight) to A (%) A (%) Example 1 Raw aluminum 50 Binder 49 hexane1 2.8 0.49 mixed powder 1 of solution 2 the present invention Example 2Same as above 50 Binder 49 heptane 1 2.8 0.49 solution 2 Example 3 Sameas above 50 Binder 49 heptane 1 2.8 0.49 solution 2 Example 4 Same asabove 49 Binder 49 octane 2 2.8 0.49 solution 2 Example 5 Same as above50 Binder 49 octane 1 4.9 0.098 solution 1 Example 6 Same as above 50Binder 49 hexane 1 4.9 0.098 solution 1 Example 7 Same as above 50Binder 49 pentane 1 4.7 1.96 solution 3 Example 8 Raw aluminum 50 Binder49 hexane 1 4.9 0.098 mixed powder 2 of solution 1 the present inventionExample 9 Raw aluminum 50 Binder 49 hexane 1 4.9 0.098 mixed powder 3 ofsolution 1 the present invention Example 10 Raw aluminum 50 Binder 49pentane 1 4.9 0.098 mixed powder 4 of solution 1 the present inventionExample 11 Raw aluminum 50 Binder 49 heptane 1 4.9 0.098 mixed powder 5of solution 1 the present invention Example 12 Raw aluminum 50 Binder 49heptane 1 4.9 0.098 mixed powder 6 of solution 1 the present inventionExample 13 Raw aluminum 50 Binder 49 octane 1 4.9 0.098 mixed powder 7of solution 1 the present invention Example 14 Raw aluminum 50 Binder 49octane 1 4.9 0.098 mixed powder 8 of solution 1 the present inventionExample 15 Raw aluminum 50 Binder 49 pentane 1 4.9 0.098 mixed powder 9of solution 1 the present invention Example 16 Raw aluminum 50 Binder 49octane 1 4.9 0.098 mixed powder 10 of solution 1 the present inventionConditions of producing formed object prior to sintering Process ofadjusting dimensions of air bubbles uniformly Thickness of formedcoating Moisture Holding time Drying process (mm) Temperature (° C.) (%)(minute) Temperature (° C.) Holding time (minute) Example 1 0.35 35 9020 70 50 Example 2 0.35 35 90 20 70 50 Example 3 0.35 35 90 20 70 50Example 4 0.35 35 90 40 70 50 Example 5 0.2 35 90 20 70 50 Example 6 0.235 90 20 70 50 Example 7 0.2 35 90 20 70 50 Example 8 0.2 35 90 20 70 50Example 9 0.2 35 90 20 70 50 Example 10 0.2 35 90 20 70 50 Example 110.2 35 90 20 70 50 Example 12 0.2 35 90 20 70 50 Example 13 0.2 35 90 2070 50 Example 14 0.2 35 90 20 70 50 Example 15 0.2 35 90 20 70 50Example 16 0.2 35 90 20 70 50 Heating conditions Degreasing processSintering process Atmosphere Temperature (° C.) Holding time (minute)Atmosphere Temperature (° C.) Holding time (minute) Example 1 Ar 520 30Ar 683 30 Example 2 Ar 520 30 Ar 650 30 Example 3 Ar 520 30 Ar 683 30Example 4 Ar 520 30 Ar 675 30 Example 5 Ar 520 30 Ar 670 30 Example 6Air 350 30 Ar 670 30 Example 7 Air 350 30 Ar 670 30 Example 8 Air 350 30Ar 670 30 Example 9 Air 350 30 Ar 670 30 Example 10 Air 350 30 Ar 670 30Example 11 Air 350 30 Ar 670 30 Example 12 Air 350 30 Ar 670 30 Example13 Air 350 30 Ar 670 30 Example 14 Ar 520 30 Ar 655 30 Example 15 Ar 52030 Ar 651 30 Example 16 Air 350 30 Ar 670 30

Next, polyethylene sheets were prepared of which the surfaces werecoated with separating compound, and the viscous compositions ofExamples 1 to 16 were coated and extended on the surface of thepolyethylene sheet by the doctor blade method, and the temperature andthe moisture were controlled to be predetermined values for a specifictime period so as to adjust the dimensions of air bubbles uniformly.Then, the viscous compositions were dried at 70° C. in an air dryer. Thecoating thicknesses of the viscous compositions, temperatures,moistures, and holding times at that time are shown in Table 3.Thereafter, the dried viscous compositions were peeled off from thepolyethylene sheets and cut out into circular shapes having diameters of100 mm to obtain formed objects prior to sintering in Examples 1 to 16.

Then, zirconia spinkle powder was spread on an alumina setter, and theformed objects prior to sintering in Examples 1 to 16 were placed on thealumina setter. The formed objects prior to sintering in Examples 1 to16 were subjected to debinding in an atmosphere where argon flowed or inair. Thereafter, the formed objects prior to sintering in Examples 1 to16 were heated to obtain foamed aluminums. The heating temperatures andheating holding times are also shown in Table 3.

Next, the contraction percentages and porosities of the obtained foamedaluminums in Example 1 to 16 were calculated. In addition, the number ofthree-dimensional pores was measured in a stereoscopic microscopephotograph, and the number of pores in the skeletons was measured in ascanning electron microscope (SEM) photograph. The obtained SEMphotograph was observed to confirm whether solidification of liquiddroplets occurred. Moreover, surface analyses were conducted by anelectron probe microanalyzer (EPMA) to confirm whether Al—Ti compoundexisted on the surface of the skeletons of the foamed aluminums. Theresults are shown in Table 5, the SEM photograph of the foamed aluminumsin Example 1 is shown in FIG. 1, and a partially enlarged photographthereof is shown in FIG. 2.

Next, rolling extension tests were performed on the foamed aluminums inExamples 1 to 16 at a rolling reduction rate of 20%, and whethercracking occurred was visually confirmed. Thereafter, rectangularsamples having dimensions of 20 mm×50 mm were cut out from the foamedaluminums, and the electrical resistances between opposed corners weremeasured. Then, the rectangular samples of the foamed aluminums werewound around an outer circumference of a cylindrical object having adiameter of 5 mm, and whether cracking occurred was visually confirmed.The results are shown in Table 5.

Comparative Examples 1 to 9

Comparative raw aluminum mixed powders 31 to 35 were prepared by usingthe same Al powder, Ti powder, and TiH₂ powder as those in Examples,Either one of the comparative raw aluminum mixed powders 31 to 35 andthe raw aluminum mixed powder 1 of the present invention was mixed andkneaded with either one of the binder solutions 1 to 5 shown in Table 2and the non-water-soluble hydrocarbon system organic solvent at themixing ratios shown in Table 4. Other conditions were same as those inExamples. Thereby, foamed aluminums in Comparative Examples 1 to 9 wereproduced. The foamed aluminums in Comparative Examples 1 to 9 wereevaluated by the same methods as those for Examples, The evaluationresults are shown in Table 5, and an SEM photograph of the foamedaluminum in Comparative Example 1 is shown in FIG. 3.

TABLE 4 Components of viscous composition Non-water-soluble Ratio ofhydrocarbon system waret- Raw aluminum mixed powder A Binder solutionorganic solvent soluble Ratio of Mixing ratio Mixing ratio Mixing ratioresin coupler surfactant to Type (% by weight) Type (% by weight) Type(% by weight) to A (%) A (%) Comparative Raw aluminum 50 Binder 49hexane 1 2.8 0.49 Example 1 mixed powder 1 of solution 2 the presentinvention Comparative Same as above 50 Binder 49 heptane 1 2.8 0.49Example 2 solution 2 Comparative Same as above 50 Binder 49 octane 18.82*⁴ 0.49 Example 3 solution 4 Comparative Same as above 49 Binder 49pentane 1 4.9 4.9*⁵ Example 4 solution 5 Comparative Comparative raw 50Binder 49 pentane 1 4.9 0.098 Example 5 aluminum mixed solution 1 powder31 Comparative Comparative raw 50 Binder 49 hexane 1 4.9 0.098 Example 6aluminum mixed solution 1 powder 32 Comparative Comparative raw 50Binder 49 heptane 1 4.7 0.098 Example 7 aluminum mixed solution 1 powder33 Comparative Comparative raw 50 Binder 49 octane 1 4.9 0.098 Example 8aluminum mixed solution 1 powder 34 Comparative Comparative raw 50Binder 49 pentane 1 4.9 0.098 Example 9 aluminum mixed solution 1 powder35 Condition of producing formed object prior to sintering Process ofadjusting dimensions of air bubbles uniformly Thickness of formedcoating Moisture Holding time Drying process (mm) Temperature (° C.) (%)(minute) Temperature (° C.) Holding time (minute) Comparative 0.35 35 9020 70 50 Example 1 Comparative 0.35 35 90 20 70 50 Example 2 Comparative0.35 35 90 20 70 50 Example 3 Comparative 0.2 35 90 20 70 50 Example 4Comparative 0.2 35 90 20 70 50 Example 5 Comparative 0.2 35 90 20 70 50Example 6 Comparative 0.2 35 90 20 70 50 Example 7 Comparative 0.2 35 9020 70 50 Example 8 Comparative 0.2 35 90 20 70 50 Example 9 Heatingconditions Degreasing process Sintering process Atmosphere Temperature(° C.) Holding time (minute) Atmosphere Temperature (° C.) Holding time(minute) Comparative Ar 520 30 Ar   690*³ 30 Example 1 Comparative Ar520 30 Ar   620*³ 30 Example 2 Comparative Ar 520 30 Ar 683 30 Example 3Comparative Ar 520 30 Ar 670 30 Example 4 Comparative Air 350 30 Ar 67030 Example 5 Comparative Air 350 30 Ar 670 30 Example 6 Comparative Air350 30 Ar 670 30 Example 7 Comparative Air 350 30 Ar 670 30 Example 8Comparative Air 350 30 Ar 670 30 Example 9 *⁴out of the scope of Claim 7*⁵out of the scope of Claim 8 *³out of the scope of Claim 1

TABLE 5 Evaluation of current collector for positive electrode oflithium-ion Evaluation of foamed aluminum battery Presence or MinimumPresence or Presence absence of diameter at Number of pores absence ofor absence cracking Filling which active in skeleton per solidified ofAl—Ti after 10% density material does Number of skeleton length aluminumin compound Electric rolling and of active not fall in three-dimensionalof 100 μm the form of on skeleton resistivity 5 mmφ material windingtest pores (PPI*¹⁾ (pores/100 μm) liquid droplet surface (×10⁻⁶ Ωm)winding test (g/cm³) (mmφ) Example 1 52 2.9 Absent Present 3.1 Absent4.8 2 Example 2 52 3.5 Absent Present 5.4 Absent 4.7 2 Example 3 52 2.2Absent Present 2.2 Absent 4.6 1.5 Example 4 65 2.3 Absent Present 2.5Absent 4.8 2 Example 5 56 2.5 Absent Present 2.6 Absent 4.2 2 Example 655 2.5 Absent Present 2.6 Absent 4.2 1.5 Example 7 77 2.7 Absent Present2.7 Absent 4.2 2 Example 8 54 2.8 Absent Present 2.9 Absent 4.3 2Example 9 55 2.3 Absent Present 2.3 Absent 4.3 2 Example 10 52 2.6Absent Present 2.8 Absent 4.2 2 Example 11 53 2.2 Absent Present 3.2Absent 4.2 2 Example 12 55 2.4 Absent Present 3.2 Absent 4.3 2 Example13 53 2.8 Absent Present 3.4 Absent 4.1 2 Example 14 55 3.4 AbsentPresent 4.9 Absent 4.1 2.5 Example 15 55 3.2 Absent Present 4.3 Absent4.2 2.5 Example 16 54 2.4 Absent Present 3.2 Absent 4.2 2 Comparative 702 Present* Present 2.9 Present* — — Example 1 Comparative 50 5.1 AbsentPresent 12.4* Present* — — Example 2 Comparative 51 4.6 Absent Present11.9* Present* — — Example 3 Comparative 65 4.3 Absent Present 11.2*Present* — — Example 4 Comparative 52 1.8* Absent Present 8.9* Present*— — Example 5 Comparative 53 5.2 Absent Absent 12.2* Present* — —Example 6 Comparative 51 2.6 Present* Present 2.4 Absent — — Example 7Comparative 51 2.2 Absent Present 2.8 Present* — — Example 8 Comparative55 1.8* Absent Present 3.1 Present* — — Example 9 Conventional 30 0Absent Absent 1.5 Absent 3.8 3.5 Example 1 *¹PPI: number of pores perinch (25.4 mm)

As can be understood from Table 5, with regard to the foamed aluminumsin Examples 1 to 16, the numbers of pores per skeleton length of 100 μmof the perforated sintered metals were in a range of 2 to 4, and thenumbers of three-dimensional pores per one inch were in a range of 52 ormore, that is, the numbers of the three-dimensional pores per onecentimeter in the metal skeletons were in a range of 20 or more. Inaddition, no lumps of liquid droplets were generated in the foamedaluminums, the electrical resistances were low, and no cracking due tothe winding test were observed. Accordingly, the foamed aluminums inExamples 1 to 16 are suitable as a current collector for a positiveelectrode of a battery or a capacitor which requires high output andhigh energy density.

Next, a lithium cobalt oxide (LiCoO₂) powder as an active material,polyvinylidene fluoride (PVdE) as a coupler, artificial graphite powderas a conductive material were mixed at a ratio by weight of 86:6:8 toprepare a cathode material. N-methyl-2 pyrrolidone as a solvent wasmixed with the cathode material to prepare a cathode active materialslurry.

Then, the foamed aluminums in Examples 1 to 16 and foamed aluminum inConventional Example 1 were immersed into this cathode active materialslurry for 10 minutes. The foamed aluminums were taken therefrom, anddried. Thereafter, the foamed aluminums were rolled to produce cathodesof lithium-ion batteries in Examples 1 to 16 having thicknesses of 0.5mm.

Here, as the foamed aluminum in Conventional Example 1, foamed aluminumof PPI was used. The foamed aluminum was produced by a method ofpressing aluminum into a casting mold having a core of sponge urethanewhich is mentioned as the second method in the related art. In addition,the filling densities of the cathode active materials of the foamedaluminum in Examples 1 to 16 and the foamed aluminum in ConventionalExample 1 are shown in Table 5.

Then, cylindrical objects having diameters of 1 mm, 1.5 mm, 2 mm, 2.5mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, and 5 mm were respectively prepared. Thecathodes of lithium-ion batteries in Examples 1 to 16 and ConventionalExample 1 were wound. Whether or not the active materials were peeledoff was visually observed, and the minimum diameters with which peelingwere not observed are shown in Table 5.

As can be understood from the results in Table 5, with regard to thecathodes of the lithium-ion batteries in Examples 1 to 16, the activematerials were not peeled off even in the case where the cathodes werewound around the cylindrical objects having diameters of 1.5 mm to 2.5mm. On the other hand, with regard to the cathode in ConventionalExample 1, the active material was peeled off when the cathode was woundaround the cylindrical object having a diameter of 3 mm. In addition,the active material filling density of the cathode of the lithium-ionbatteries in Examples 1 to 16 were in a range of 4.1 g/cm³ or greater.In contrast, the active material filling density of the cathode inConventional Example 1 was 3,841 g/cm³, which was small.

INDUSTRIAL APPLICABILITY

The present invention can be applied as a method for producing a currentcollector of a lithium-ion secondary battery or an electrical doublelayer capacitor as well as a method for producing foamed aluminum.

1. A method for producing porous sintered aluminum comprising: mixingaluminum powder with a sintering aid powder containing titanium toobtain a raw aluminum mixed powder; mixing the raw aluminum mixed powderwith a water-soluble resin binder, water, and a plasticizer containingat least one selected from polyhydric alcohols, ethers, and esters toobtain a viscous composition; drying the viscous composition in a statewhere air bubbles are mixed therein to obtain a formed object prior tosintering; and heating the formed object prior to sintering in anon-oxidizing atmosphere, wherein when a temperature at which the rawaluminum mixed powder starts to melt is expressed as Tm (° C.), atemperature T (° C.) of the heating fulfills Tm−10 (° C.)≦T≦685 (° C.).2. The method for producing porous sintered aluminum according to claim1, wherein an average particle diameter of the aluminum powder is in arange of 2 to 200 μm.
 3. The method for producing porous sinteredaluminum according to claim 1, wherein when an average particle diameterof the sintering aid powder is expressed as r (μm), and a mixing ratioof the sintering aid powder is expressed as W (% by mass), r and Wfulfill 1 (μm)≦r≦30 (μm), 1 (% by mass)≦W≦20 (% by mass), and 0.1≦W/r≦2.4. The method for producing porous sintered aluminum according to claim1, wherein the sintering aid powder is either one or both of titaniumand titanium hydride.
 5. The method for producing porous sinteredaluminum according to claim 1, wherein non-water-soluble hydrocarbonsystem organic solvent containing 5 to 8 carbons is added to the viscouscomposition.
 6. The method for producing porous sintered aluminumaccording to claim 1, wherein the water-soluble resin binder iscontained at a content in a range of 0.5% to 7% of a quantity (mass) ofthe raw aluminum mixed powder.
 7. The method for producing poroussintered aluminum according to claim 1, wherein surfactant is added tothe raw aluminum mixed powder at a content in a range of 0.02 to 3% of aquantity (mass) of the raw aluminum mixed powder.
 8. The method forproducing porous sintered aluminum according to claim 1, wherein theviscous composition is extended to have a thickness of 0.05 mm to 5 mm,and then the viscous composition is dried to produce the formed objectprior to sintering as a plate-like formed object.
 9. Porous sinteredaluminum comprising: metal skeletons having a three-dimensional networkstructure of perforated sintered metal, wherein pores exist between themetal skeletons, Al—Ti compound is dispersed in the perforated sinteredmetal, and the pores are formed at an amount of 20 or more pores perlinear length of 1 cm, and thereby, an overall porosity is in a range of70 to 99%.